Since saturated polyester represented by polyalkylene terephthalate is excellent in characteristics such as strength, transparency, heat resistance and chemical resistance, it has been used in various applications such as fibers, a film, a bottle and others.
In recent years, however, a great problem has been arisen with respect to recycling of spent saturated polyester occurring in an increasing amount or of a disqualified product occurring in production process for saturated polymer, along with an increase in use amount thereof from the viewpoint of the global environment. Recycling methods for saturated polyester currently adopted are classified into three methods in a broad sense, that is a material recycling method, a thermal recycling method and a chemical recycling method.
As the material recycling method, there have been known a method melting saturated polyester to mix it with a virgin raw material to reuse the mixture, a method using only a recovered product to produce a low grade product and the like method. Since a recycled product obtained by means of the material recycling method is low in strength, a necessity arises for a measure to mix a recovered product with a virgin raw material. Since mixing of another plastic into saturated polyester has a problem to greatly reduce strength of a recycled product or color it, a necessity arises for selecting only saturated polyester strictly prior to melting.
While the thermal recycling method reuses heat generated in burning recovered saturated polyester instead of a fuel, saturated polyester is burned in the method; therefore, it is not recycling and a problem arises in an environmental aspect because of generation carbon dioxide in combustion.
The chemical recycling method converts an abandoned saturated polyester product back to a raw material to again synthesize saturated polyester from the raw material, which enables recycling a resource that is an essential object, and which facilitates solving of the problems such as contamination, mixing-in of different resin, reduction in molecular weight in molding, thermal coloration; therefore it is considered to be the best recycling method. Examples of the chemical recycling method include: (1) a hydrolysis method in which saturated polyester is heat-treated together with a strong acid or alkali aqueous solution to thereby convert the saturated polyester to phthalic acid as a dicarboxylic acid such as terephthalic acid or a salt thereof for recovery, (2) a methanol decomposition method in which saturated polyester is heat treated together with methanol to convert it dicarboxylic acid dimethyl ester such as dimethyl terephthalate for recovery and (3) a glycol decomposition method in which saturated polyester is heat treated together with a glycol such as ethylene glycol or propylene glycol and thereby depolymerized to recover prepolymers of saturated polyester. Examples thereof include: a method described in JP-A No. 11-302208 using sodium hydroxide as a catalyst, a method described in JP-A No. 11-322677 in which glycol decomposition is followed by methanol decomposition, a method described in JP-A No. 2000-53802 or 2000-169623 regenerating polyethylene terephthalate, a method described in JP-A No. 2000-191766 using a titanium compound or a tin compound as a catalyst, a method described in JP-A No. 2000-198876 in which plastics other than polyethylene terephthalate are separated off and a method described in JP-A No. 2000-302707 using iron oxide as a catalyst.
In a chemical recycling method for saturated polyester currently available, however, high temperature in the vicinity of 200° C. is necessary in order to advance an ester exchange reaction, which therefore has led to a problem to be unpreferable in regard to energy consumption. In a case where an ester exchange reaction is performed using a low-molecular-weight alcohol, the reaction is often performed under high pressure in order to prevent the alcohol to be evaporated off, having led to problem of a need for an expensive high pressure vessel.